The present invention relates generally to improvements in unloading systems for hoppers, and specifically to unloading systems for hoppers adapted to carry granular lading such as plastic pellets that tend to collect in and jam the unloading system.
Hopper unloading systems provided heretofore all have a valve disposed between the hopper and a discharge tube that is adapted to be connected to a source of air under pressure or to a source of reduced pressure or vacuum. Such valves all include a movable valve member which selectively opens and closes an opening between the hopper and the discharge tube.
One form of such prior structure is illustrated in U.S. Pat. No. 2,650,726 granted Sept. 1, 1953 to Edmund R. Aller et al. That patent utilized a rotary valve member to open and close the opening between the hopper and the discharge tube. Although perfectly satisfactory for most forms of lading, it was found that granular materials such as plastic pellets tend to collect in the space between the stationary valve member and the rotary valve member, and tend to wedge the parts so tightly that the rotary valve member cannot be moved.
The same type of problem is encountered when sliding valve members are utilized rather than the rotary valve members described above. An example of a sliding valve member is illustrated in U.S. Pat. No. 3,207,560 granted Sept. 21, 1965 to Wilbur A. Brown. Granular material such as plastic pellets also tend to jam or wedge these sliding valves because the pellets tend to collect in the space at the bottom of the discharge tube between the movable valve member and the discharge tube, these particles wedging so tightly that the movable valve member cannot be moved. Attention is also directed to U.S. Pat. No. 3,637,262 granted Jan. 25, 1972 to Franklin P. Adler which shows a similar arrangement.
These prior unloading systems also had the disadvantage that granular material remained wedged in the valve structure after unloading of the lading from the hopper and such retained granular material was often discharged into the next lading thus to contaminate subsequent lading. Certain of the prior unloading systems also allowed leakage of air into or out of the system or entry of dirt or water into the interior of the system during the unloading, all of which is objectionable.